Phosphorus (P) or boron (B) atoms can be doped at temperatures as low as 80 to 350 °C, when crystalline silicon (c-Si) is exposed only for a few minutes to species generated by catalytic cracking reaction of phosphine (PH3) or diborane (B2H6) with heated tungsten (W) catalyzer. This paper is to investigate systematically this novel doping method, “Cat-doping”, in detail. The electrical properties of P or B doped layers are studied by the Van der Pauw method based on the Hall effects measurement. The profiles of P or B atoms in c-Si are observed by secondary ion mass spectrometry mainly from back side of samples to eliminate knock-on effects. It is confirmed that the surface of p-type c-Si is converted to n-type by P Cat-doping at 80 °C, and similarly, that of n-type c-Si is to p-type by B Cat-doping. The doping depth is as shallow as 5 nm or less and the electrically activated doping concentration is 1018 to 1019 cm-3 for both P and B doping. It is also found that the surface potential of c-Si is controlled by the shallow Cat-doping and that the surface recombination velocity of minority carriers in c-Si can be enormously lowered by this potential control.

To reduce surface recombination at an amorphous silicon (a-Si)/crystalline silicon (c-Si) interface in heterojunction solar cells, a thin phosphorus-doped back surface field (BSF) layer is applied to c-Si. Thin BSF layers are doped at temperatures lower than 350 °C by radical doping. The reduction in the surface recombination velocity of n-type c-Si is investigated by comparing the effective minority carrier lifetimes of c-Si samples with and without doping. Using radical-doped BSF layers, the effective minority carrier lifetimes of the samples with the thin intrinsic a-Si passivation layers increase significantly. The change in effective minority carrier lifetime under the BSF layer doping condition is also investigated. An effective minority carrier lifetime of 1.6 ms is observed in the radical-doped sample with the 6-nm-thick intrinsic a-Si passivation layer. The high carrier concentration of the radical-doped BSF layers can also decrease the contact resistivity to a metal electrode. Therefore, the radical-doped BSF layers can be utilized for passivation and ohmic contact formation on the back surface of the heterojunction solar cells.

We have discovered that phosphorus (P) atoms can be doped into crystalline silicon (c-Si) at temperatures below 350 o C or even at 80 o C by using species generated by catalytic cracking reaction of phosphine (PH 3) molecules with heated tungsten (W) catalyzer in Cat-CVD apparatus. As further investigation, here, we study the feasibility of low temperature doping of boron (B) atoms into c-Si by using decomposed species generated similarly from diborane (B 2 H 6) molecules. Dependency of properties of doped layers on catalyzer temperature (T cat) and substrate temperature (T s) is studied by both the Van der Pauw method based on the Hall-effect measurements and secondary ion mass spectroscopy (SIMS) for B doping in addition to P doping. It is found that, similarly to P doping, the surface of n-type c-Si is converted to p-type even at T s =80 o C for T cat over 800 o C when c-Si is exposed to B 2 H 6 cracked species for a few minutes, and that the heat of substrate over 300 o C is likely to help for B doping contrary to P doping.

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